The difference between matching and challenge grants was not statistically significant, actually. More generally, that study's evidence is suggestive at best; it was underpowered (couldn't have distinguished a 30% increase in donations from noise) and didn't correct for multiple (12 in the field, 10 in the lab) hypothesis tests. They also mis-described what a p-value means, which doesn't directly invalidate their results but makes me pretty generally worried.